correct some missing-prototype warnings

This avoids the following warnings:
no previous prototype for 'bl2_arch_setup' [-Wmissing-prototypes]
no previous prototype for 'plat_log_get_prefix' [-Wmissing-prototypes]

Also correct a compilation issue if BL2_IN_XIP_MEM is enabled:
uintptr_t is not defined.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/plat/common/plat_log_common.c b/plat/common/plat_log_common.c
index 49e1c15..c757c6b 100644
--- a/plat/common/plat_log_common.c
+++ b/plat/common/plat_log_common.c
@@ -5,6 +5,7 @@
  */
 
 #include <debug.h>
+#include <platform.h>
 
 /* Allow platforms to override the log prefix string */
 #pragma weak plat_log_get_prefix